Bar codes have been used in a wide variety of applications as a source for information. Typically bar codes are scanned at a point-of-sale terminal in merchandising for pricing and inventory control. Bar codes are also used in controlled personnel access systems, and in manufacturing for work-in process and inventory control systems, etc. The bar codes themselves represent alphanumeric characters by series of adjacent stripes (bars and spaces) of different widths and reflectivity, such as in the universal product bar code (UPC) used in retail merchandising.
Bar code reading systems or scanners have been developed to read bar codes. In many such scanners, a laser diode emits a light beam. The light beam is scanned across the bar code and return light from the bar code is collected by the scanner. The intensity of the return light is proportional to the reflectance of the area illuminated by the laser diode. This return light is converted into an electric signal using a photodiode. The signal is then amplified, digitized and decoded.
During normal operation of the scanner there are periods of time in which the scanner is inactive or idle. The laser diode can be turned off during these inactive or idle periods to conserve power and reduce heating which is potentially damaging to the laser diode. Conservation of power increases battery life in portable and hand held scanners and data collection terminals incorporating such scanners, then known as scan engines.
Laser diodes are operated by the electrical current provided by drive and control systems or circuits, which may provide optical power regulation by control of current through the laser diode, see, e.g. Eastman et al, U.S. Pat. No. 5,200,597 issued Apr. 6, 1993 and U.S. Pat. Appl. Ser. No. 08/296,788 filed Aug. 26, 1994 and assigned to the assignee of this application. Typically an optical power regulator monitors the current through a monitor photodiode that is housed in the same package as the laser, in such a manner as to be illuminated with a portion of the laser's emitted light. The monitor photodiode current is compared with a predetermined value, corresponding to the desired optical power, and the difference between these values is amplified and drives the laser, thus forming a closed-loop control system. Laser diodes require an operating voltage which exceeds a threshold, called the forward voltage drop, in order to conduct current. When the laser diode operating voltage nears the forward voltage drop of the laser diode, momentary power fluctuations can cause the laser diode to stop conducting, because the operating voltage is less than the forward voltage drop. During these low voltage periods, a conventional optical power regulator senses the lack of optical output power because the photodiode current decreases or cuts off. Since the optical power then drops below its desired value, the regulator attempts to increase the optical power by increasing the laser current. However, because the operating voltage is too low to exceed the forward voltage drop of the laser diode, the regulator saturates without effecting an increase in optical power output from the laser. When voltage is later restored, the laser immediately begins conduction and the saturated power regulator causes a current surge through the laser diode that can permanently damage the laser diode. The foregoing problem is exacerbated when the optical power regulator is fabricated with power handling field effect transistors (FETS), made using bulk CMOS (complimentary metal oxide silicon) fabrication techniques and then called MOSFETS, because power FETS tend to introduce forward voltage drops which the operating voltage must exceed in addition to the laser diode forward voltage drops.